In recent years there has been growing interest in organic electronic (OE) devices, for example organic field effect transistors (OFET) for use in backplanes of display devices or logic capable circuits, and organic photovoltaic (OPV) devices. A conventional OFET has a gate electrode, a gate insulator layer made of a dielectric material (also referred to as “dielectric” or “gate dielectric”), source and drain electrodes, a semiconducting layer made of an organic semiconductor (OSC) material, and typically a passivation layer on top of the aforementioned layers to provide protection against environmental influence or against damage from subsequent device manufacturing steps.
For many top gate (TG) OFET applications fluorodielectrics like Cytop® or Hyflon® polymers are usually deposited from fluorosolvents, to provide a dielectric layer with inherent low permittivity (“low-k”) and good orthogonality to a number of OSC material classes. However, the use of such dielectrics has a number of drawbacks, with the main issue being lack of cross-linking functionality which is difficult to incorporate without modifying the permittivity. In addition, the use of fluorinated solvents is not preferred for environmental and cost reasons.
It is desirable and advantageous to incorporate a reactive functionality into a dielectric which enables photo-patterning, as not only does this provide orthogonality to solvents employed in subsequent processing steps, but also allows for stack integration by the formation of via interconnects to the source and drain electrodes (S/D) below the dielectric and OSC layers, using photolithographic processes.
In developing a crosslinkable top gate dielectric, the bulk properties of the material used, such as low k and solubility must be retained. It is also important to consider the functional group employed in the cross-linking reaction to ensure quantities of polar or charged by-product species are not generated as side products. Such species will become incorporated into the cross-linked dielectric, causing unwanted effects such as an increase in the k value, or the introduction of charged species may dope the OSC layer resulting in decreased performance and stability.
Thus there is a need for solution processable dielectrics for the preparation of dielectric layers in OE devices, which have low permittivity, show good orthogonality to a number of OSC material classes, can be deposited from non-halogenated solvents, can be cross-linked to enable photo-patterning and stack integration by the formation of via interconnects to the S/D electrodes, and wherein the cross-linking reaction does not generate undesired polar or charged by-product species as side products.
In particular, it is desired to have a UV crosslinkable dielectric with a solubility profile orthogonal to the OSC polymer, a permittivity as low as possible to give the highest TFT performance, and a cross-linking functional group, and/or cross-linking agents, incorporated to allow via interconnect patterning. The dielectric should, in conjunction with the OSC polymer, give high mobility, high on currents, and low off currents.
The present invention enables satisfying the above needs by using a polycycloolefinic polymer in the dielectric layer of an OE device which does not contain a polar or aromatic unit, is chemically inert, has an inherently low k, has a pendant group that imparts solubility and cross-linking functionality, and is selected from pendant olefinic chains wherein preferably the isomerization of the C—C double bond in the olefinic chain can be controlled.